elasticsearch/docs/reference/modules/discovery/no-master-block.asciidoc

[[no-master-block]]
=== No master block settings

For the cluster to be fully operational, it must have an active master.  The
`discovery.zen.no_master_block` settings controls what operations should be
rejected when there is no active master.

The `discovery.zen.no_master_block` setting has two valid values:

[horizontal]
`all`:: All operations on the node--i.e. both read & writes--will be rejected.
This also applies for api cluster state read or write operations, like the get
index settings, put mapping and cluster state api.
`write`:: (default) Write operations will be rejected. Read operations will
succeed, based on the last known cluster configuration.  This may result in
partial reads of stale data as this node may be isolated from the rest of the
cluster.

The `discovery.zen.no_master_block` setting doesn't apply to nodes-based APIs
(for example cluster stats, node info, and node stats APIs). Requests to these
APIs will not be blocked and can run on any available node.